Visit any top porn site today, and you'll be spoiled for choice. At YouPorn (which Wikipedia brands the most-popular "adult" site in the world), the viewer can choose from thousands of videos in dozens of genres. But for as long as we've had Internet porn, one category has stood as the most sought-after of all: barely legal teens.

Across the web, videos and images featuring 18- and 19-year-olds — or actresses in their twenties trying to look younger — are by every measure the most in demand. "Teen porn" is the most common genre-specific term used in Google searches, and teen-themed videos dominate the top 25 most-viewed videos on YouPorn. (Link is absolutely NSFW.)

This particular fascination with very young women predates the Internet. Playboy began its famous college girls series in the 1970s, focusing on the "girls" of one particular athletic conference each fall. The chosen coeds were not much younger than the magazine's centerfolds, though the magazine was careful to emphasize the particularly amateur appeal of the models from the Big Ten or Southeastern Conference. The genre really took off in 1993, just before most Americans began to go online, when Larry Flynt launched his Barely Legal magazine.

The text on the cover of that first issue was characteristic of the Hustler publisher's penchant for bluntness, promising "women who were girls just yesterday" and "a celebration of sexual debutantes." But nothing was more telling than the title itself, which whispered the transgressive thrill of staring at the bodies of young women just days removed from being "jailbait." Where most adult magazines were eager to distance themselves from child pornography, Hustler reveled in the close association with the illicit and the illegal. The implication was clear: most men would really like to be looking at naked women who were well under 18. Rather than shame them for that longing, Larry Flynt was going to give them the next best (legal) thing. Within a few short years, countless porn-site operators would borrow that same sly marketing tactic, winking indulgently at the ephebophilia of their visitors.

The extraordinary popularity of the barely legal genre (Flynt couldn't trademark the catchphrase) raises an obvious question: Why are so many straight adult men so turned on by women at or below the age of consent? The answer for many — across the ideological spectrum — takes some form of an appeal to "nature." The well-known right-wing columnist John Derbyshire wrote in The National Review in 2005 that "beyond our salad days, very few of us are interesting to look at in the buff. Added to that sadness is the very unfair truth that a woman's salad days are shorter than a man's — really, in this precise context, only from about 15 to 20." That half or more of a woman's sexual "salad days" would pass before she was a legal adult was, for Derbyshire and nearly all of his entirely un-outraged fellow columnists at America's best-known conservative magazine, just "a sad truth about human life."

Beyond Derbyshire, the most common explanation given for adult men's particularly intense attraction to teen girls is reproduction. But on closer scrutiny that theory falls apart. Women's fertility peaks between 22 and 26, well after their "salad days" have come to a close. Barely and not-yet legal teens alike have statistically higher rates of complications in pregnancy than women in their twenties. From a medical historical perspective, there has never been a time when 17 year-olds were more fertile than women just five or six years older. The argument that men in their 30s, 40s, and beyond are evolutionarily hardwired to lust after girls just above or below the adulthood threshold has less merit than we think.

One alternative answer has much more to do with adult men's anxiety than with their reproductive longings. In the fantasy world of "barely legal" pornography, the teen girl is an ingénue longing for sexual initiation at the hands and body of an experienced older man. For an older man (the average male porn user is over 30) perhaps intimidated by the erotic and emotional demands of his own female peers, the imagined naïveté of a much-younger woman is a source of comfort. The less experience she has, the less likely she'll mock his clumsiness and the more likely she'll appreciate whatever savoir-faire he does possess.

Still another explanation for the extraordinary popularity of the genre is adult men's longing to re-imagine their own unhappy sexual debut. Sex educator Lanae St. John suggested in an interview that straight adult men are drawn to teen girls to "relive an adolescence they didn't get to have. It's less predatory than it is wistful." Masturbating to images of teen girls, St. John says, is less about men's refusal to grow up and more "a tool for working through past issues." Perhaps, but how long does that "healing process" need to take? Porn may be less addictive than its detractors insist, but it's probably a less effective therapeutic aid than its champions hope.

Whatever the reasons behind adult men's troubling fascination with "jailbait," it's worth noting that the porn industry is hardly alone in eroticizing adolescents. The ongoing and increasing sexualization of underage girls is widespread in fashion, advertising, and popular culture. Whether "barely legal" porn is a cause or a mere byproduct of the youth-sexualization phenomenon is unclear. What is clear is that for a great many men online, the images they want most to see are of the disconcertingly young.

It would be nice to believe that the hunt for barely legal porn has no impact on these men's sexual lives with real, flesh-and-blood, post-salad-day adult women. For some guys, it may be possible to masturbate regularly to images of 20-year-olds in high school cheerleader outfits without any loss of passion for their 30- or 40-something partner. But I'm leery of that claim. The reality is that only those who are wise and confident enough to challenge us can help us grow. Age isn't just a number; that confidence and wisdom takes time to emerge. So when men eroticize the young, the tentative, and the innocent — for whatever reason — they're possibly just eroticizing their own reluctance to accept adulthood and responsibility. In that scenario, everybody loses.

Hugo Schwyzer is a professor of gender studies and history at Pasadena City College and a nationally-known speaker on sex, relationships, and masculinity. You can see more of his work at his eponymous site.